Malignant fibrous histiocytoma originating from the thymus

A 63-year-old man with a history of acute coronary syndrome presented with anterior chest pain that had lasted more than 3 months. During treatment for acute myocardial infarction on admission, computed tomography (CT) was performed incidentally, revealing an anterior mediastinal tumor 28 mm in diameter. CT findings after 3 months demonstrated an anterior soft tissue mass involving the left brachiocephalic vein and showing rapid growth. Radical resection of the tumor with thymectomy was performed. Histological examination showed storiform-pleomorphic malignant fibrous histiocytoma (MFH) originating from the thymus, and Hassall's bodies were identified. Immunohistochemical staining showed tumor cells with positive results for vimentin and CD68. The Ki-67 index (MIB-1) was >50%, but other markers such as cytokeratin AE1/AE3, smooth muscle actin, S-100 protein, CD34, and CD45 were negative. Mediastinum MFH has been reported in 34 cases, but this appears to be a rare description of MFH originating from the thymus.